The present invention relates to the field of video image processing, and in particular to correcting the phase of a vertically distorted digital picture, such as a digital television picture.
Television signals are often displayed using interlaced scanning. As is shown in FIG. 4, the interlaced scanning method is characterized in that the picture being displayed (full picture) is decomposed into lines, such that the lines of this full picture are divided into two temporally sequential half pictures. The lines with an originally odd line number are displayed in the first half picture, and the lines with an originally even line number are displayed in the second half picture. The standard for displaying the picture is specified in the guideline ITU-R 601.
As shown in FIG. 4, the number of lines of each of the two half pictures is half the number of lines of the full picture. A vertical decimation factor (VDEC) can be defined for the interlaced scanning method. This factor corresponds to the ratio of the number of lines of the full picture to the number of lines of the half pictures. In the example of FIG. 4, the vertical decimation factor is two (2).
FIG. 4 also shows that a phase offset PHS exists between the lines of the first half picture and those of the second half picture. This phase offset amounts to one line. In general, the phase offset is calculated as follows:
                    PHS        =                  VDEC          2                                    (        1        )            This phase offset must be taken into account at the beginning, when generating the second picture.
The preceding description assumes a vertical decimation factor VDEC that is constant over the full picture or the half picture. However, for certain applications and for displaying certain effects, it may be desirable to create digital pictures by the interlaced scanning method, with a vertical distortion that changes as a function of vertical position. For example, expansion or compression of a picture or the display of a vertical panorama effect requires a change of the vertical decimation factor or expansion factor over the picture. The vertical decimation factor VDEC thus becomes a function of the line of the half picture, i.e. VDEC=VDEC(L), where L designates the particular line of the half picture. For displaying other effects, it is also conceivable to make the vertical decimation factor also dependent on other parameters, for example the pixel position, et cetera. However, to calculate a phase correction factor for the second half picture, so as to take into account the phase offset between the second half picture and the first half picture, only the vertical decimation factor VDEC on the line is relevant. The following therefore holds for the phase offset:
                              PHS          ⁡                      (            1            )                          =                              VDEC            ⁡                          (              1              )                                2                                    (        2        )            It is necessary to perform a phase correction for the second half picture, since otherwise an undesirable picture distortion, which is perceived as interference, occurs in the vertical direction during the course of the picture.
There is a need for a system and method of correcting the phase of a vertically distorted digital picture.